When 'gt mq submit' is called:
1. Parse source issue from branch
2. Check if issue has a parent epic
3. Check if integration/<epic> branch exists
4. If yes: set target=integration/<epic>
5. If no: set target=main
The --epic flag still works for explicit targeting.
Closes gt-h5n.7
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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